The invention relates to an implantable defibrillation system with an intracardial or subcutaneous defibrillation electrode.
What is known as ventricular fibrillation is a disturbance in heart rhythm which rapidly, as a consequence of the erratic contraction of individual muscle fibers and the resultant uncoordinated heart activity, leads to what is known as sudden cardiac death, unless defibrillation is done by suitable stimulation provisions. Such defibrillation is initially done externally, using suitable devices that produce high-energy pulses. For the sake of cardioversion, electrodes are applied to the chest of the patient and carry the pulses into the thorax.
Since such rescue provisions, using external devices and carried out by suitably trained helpers, often come too late for many patients, implantable defibrillators have since been created. They are provided with batteries and are implanted along with the electrodes in the body of the patient. If additional detection means are provided, which automatically trip defibrillating pulses upon the occurrence of ventricular fibrillation, then the affected patient can move about, even outside the range of medical assistance, without fear of possible danger from ventricular fibrillation that might occur.
Such implantable systems are still limited in their effectiveness and service life, however, by the high energy consumption in the defibrillation. Although the required energy can be reduced considerably in implanted devices compared with external devices, nevertheless the resultant useful life is short, especially if defibrillation is done frequently, and so reimplantation soon becomes necessary.
The difficulty in the choice of suitable endothoracic electrodes is that the electrodes must be embodied with the largest possible surface area, so that the stimulating current can be distributed over the entire affected region of the heart muscle. In contrast to the normal, pacemaking heart stimulation, defibrillating pulses after local excitation are not automatically propagated in the heart. Instead, provision must be made so that all the regions to be stimulated are located within the range of action of the stimulating current.
German Patent DE 26 43 956 C2 discloses a defibrillating electrode arrangement with large-area electrodes. It is intended for epicardial application and is meant to conform to the outline of the heart, with one of the electrodes conforming to the region of the apex of the heart and the other to the region of the upper heart.
One unfavorable aspect of these electrodes is that they still require a relatively large amount of energy, so that all the excitable cells of the heart can be depolarized. Moreover, the electrodes also have a certain intrinsic rigidity, which, if they do not move along with the surface of the heart, leads on the one hand to inadequate electrical contact of the heart muscle and on the other to a hindrance of the motion of the heart, since the electrodes involved are usually permanently stitched to the heart.
European Patent Disclosure EP 0 211 166 A2 also discloses an electrode that comprises a plurality of star-like or radiating electrode arms, which are secured to the surface of the heart. Once again there is the disadvantage that securing the individual electrode arms is relatively time-consuming. Moreover, securing them locally often does not accomplish the necessary distribution of current that must in fact be attained in the defibrillation.
One disadvantage of the known electrodes is that nevertheless, relatively large amounts of energy are still needed for the defibrillation, so that either very large energy reservoirs must be provided within the implantable housing--as a result of which, because of its large volume, in the final analysis the implant cannot be manipulated like a modern implanted cardiac pacemaker--or, if there is frequent recourse to the defibrillation function, then premature depletion of the energy reservoirs must be expected. This latter is especially disadvantageous in devices of the kind in which the defibrillation device is combined with a conventional cardiac pacemaker, because then if the battery is depleted, none of the life-sustaining functions will any longer be available.